Semi-finished products in supple or rigid sheet form are already known, in which the sheets are constituted of reinforcing fibers, generally glassfibers, with a thermoplastics resin selected from polyamides, polyesters, etc. . . . but beinjg smainly a polyolein. The sheets are designed to be transformed under heat by molding-stamping or thermoshaping.
It is known that, in a thermoplastics resin, the fibers only produce their maximum reinforcing effect if:
on the one hand, they have at least a critical length of about 5 mm for glassfibers, below which the material will break under strain without the fibers breaking,
and if, on the other hand, they are completely wetted by the thermoplastics resin, a quality which cannot be obtained when the fibers are not unitary, but assembled in rove or glass strand form, since then the thermoplastics resins, in molten state, are too viscous to really impregnate them thoroughly.
It is also known that the longer the reinforcing fibers are, the poorer is the moldability of the thermoplastics semi-finished product, and that, in practice, if the length exceeds a few centimeters, the distribution of the reinforcement, although it may be homogenous within the sheet, is no longer uniform throughout the whole molded piece.
One means of individualising the reinforcing fibers, without breaking them, inside a thermoplastics resin, consists in dispersing them in aqueous phase, for example in roves or glass strands form, and in mixing them with the powdered resin. This means, which uses the wet method, comes under the general principle of papermaking and therefore will require, in order to obtain the ideal sheet, that a number of technical conditions be met:
1. A retention of the elements of the composition, in particular the powders, namely the resin and the additives (for example antioxidants, dyes, antistatic agents, and age-protecting agents).
2. A good dispersion of the reinforcing fibers with the powder, which fibers should be distributed homogeneously through the thickness of the sheets.
3. The formation of a sheet with sufficient cohesion to allow its drying and its manipulation during transformation.
Various solutions have been proposed, consisting:
either in draining the mixture of cut fibers and powder through a glass strand mat which is used as a self-supporting base, as described in U.S. Pat. No. 3,684,545
or in adding to the mixture of cut fibers and powder, a latex binder and natural or synthetic fibers with high specific surface area, as described in GB-A-1 263 812, in EP-A-0 039 292 and in U.S. Pat. No. 4,426,470.
Each case, therefore, implies bringing raw materials into the sheet, the nature and quality of which materials is undesirable for the properties of the finished plastics piece.
In some cases, a good dispersion of the reinforcing fibers is achieved by working with very high dilutions and/or by increasing the viscosity of the water as described in EP-A-0 180 863, or else by producing a foamy dispersion as described in GB-A-1 263 812, all these means implying material modifications over the conventional papermaking machines, which are often important, such as those described in FR-A-2 179 204 and U.S. Pat. No. 3,716,449.
There is therefore a desideratum to minimise the aforesaid disadvantages by providing a reinforced thermoplastics sheet which, being prepared by the wet method, from a mixture of cut fibers and powder, without any addition of latex binder being required, contains substantially only those elements which are suitable and necessary for the final plastic application.
According to the present invention there is provided a process for preparing by the wet method a reinforced thermoplastics semi-finished product in sheet form, wherein said process comprises the following steps:
(a) providing a first aqueous dispersion of a dispersing agent with reinforcing fibers,
(b) providing a second aqueous dispersion containing at least one thermoplastics polymer powder and at least one ionic polymer additive of opposite charge to that of the first dispersion,
(c) mixing the two dispersions to form a mixture,
(d) incorporating into the mixture a flocculating agent in ionic polymer form, of high molecular weight and of charge opposite to that of said mixture, and
(e) draining the resulting suspension and drying the resulting semi-finished product.
Step (e) is preferably carried out by draining the suspension on a metallic form or wire; after drying the semi-finished product is preferably heat-strengthened.
The reinforcing fibers used in the process according to the invention are fibers of which the physical structure remains of course unchanged after molding of the semi-finished product. They are, whether used on their own or in a mixture, cylindrical synthetic fibers such as carbon fibers, organic fibers with a high melting point (aramides, polyester and others), glass wool, rock wool, but the preferred reinforcement according to the invention is constituted by glassfibers. Preferably the fibers are of length from 5 to 30 mm, more preferably from 6 to 25 mm. It is preferred, too, that the reinforcing fibers have a diameter of from 10 to 20, more preferably from 10 to 16 and most preferably from 10 to 13, e.g. 12 to 13 micrometers.
Preferably the fibers are used in a proportion of from 15 to 60% , more preferably from 20 to 40% by dry weight of the semi-finished product. At much below the 15 weight % level the quantity of fibers, e.g. glass fibers, is no longer sufficient to obtain advantageous final mechanical properties and at much above 60 weight %, moldability becomes insufficient for the transforming techniques which are usually applied to the semi-finished product.
By cylindrical fibers is meant, as opposed to fibrillated fibers, those fibers which, once spun, have a regular shape and smooth surface, and therefore which will not help to cause aqueous dispersion, or create mechanical binding, or powder retention.
With the process according to the invention, it is possible to produce a reinforced thermoplastics sheet in which each component is more or less perfectly dispersed and uniformly distributed through the thickness of the sheet, and in which the thermoplastics components and the reinforcement represent for example more than 96%, preferably from 97 to 99.5%, more preferably from 98 to 99.5% and optimally about 99%, of the dry weight of the semi-finished product.
By thermoplastics component is meant the polymer or mixture of polymers constituting the thermoplastics resin, as well as the additives which may be incorporated to facilitate hot-transformation of the semi-finished sheet and to provide desirable properties in the resulting final product (therefore present as such in the semi-finished sheet).
By reinforcement is meant the reinforcing fibers and possibly any optional non-fibrous reinforcing components such as mineral fillers.
Although the process according to the invention does not require the presence of a latex binder, it nevertheless is possible, by use of the process, to produce sheets of high powder content. In one preferred embodiment the sheets produced by the process of the invention have a ratio of powder to cylindrical fibers of from 1.5 to 5.7, and more preferably from 2 to 3, although such ratio is by no means a restriction on the process which permits production of sheets having a ratio of substantially any value; the ratio will be selected by the operator to meet the properties (flow moldability; strength etc.) required in the final product derived from the sheet.
The sheet according to the invention can also contain stainless steel fibers or aluminized glassfibers, in order to make the molded product conducting, and paper pulp cellulosic fibers to lower the cost of the composition, insofar as the molding requirements of the semi-finished product permit it and provided that this does not affect the properties necessary for the plastic application of the semi-finished sheet.
The thermoplastic resin used in the process of the invention is preferably a polyolefin such as polypropylene, polyethylene or copolymers, or a mixture of polyolefins; other thermoplastics polymers suitable for use in the process of the invention include polyamides, saturated polyesters, polystyrenes and their copolymers, polyphenylene ethers, polyvinyl chlorides, polycarbonates and other technical polymers or plastics alloys or mixtures of the above, known to those skilled in the art.
The thermoplastics resin is preferably used in powder form of average granulometry less than 800 micrometers and typically from 100 to 600 microns. Typical commercially available polymer powders will of course contain small proportions of particles with diameters above and below the average. At particle sizes much above 800 or 900 microns, the polymer powders can no longer be evenly distributed through the thickness of the sheet, especially if the density of the resin is, as for the polyolefins for example, less than 1 g/cm.sup.3. In these circumstances it has been found that the particles tend to concentrate on the side of the sheet opposite to that through which the aqueous phase is drained.
Besides the above-mentioned fibers and polymer powders, the sheet produced according to the invention can also comprise, for technical or economical purposes:
other reinforcement, for example mineral fillers, glass microspheres or microballs, ground glassfibers or ceramics fibers,
as thermoplastics components, a few percent of additives to improve adherence of the resin on the reinforcement when the piece is molded (post-lubricating of the type of organosilanes, organosilicones and others, titanates, polar polymers in dispersion), synthetic polyolefin pulps of high specific area, pigments or dyes, thermal stabilizers, and any other additives known by those skilled in the art to give to plastics materials an increased fluidity in the molten state, a resistance to light, to heat, and to other environmental stresses.
The above-described mixtures of reinforcing elements and thermoplastics components preferably represent 98 to 99.5%, e.g. about 99% of the thermoplastics sheet formed by the process of the invention, the remaining part being composed of residual traces of the different additives which are necessary for preparing by the wet method of the invention a sheet which will have the required qualities.
The problems arising when producing such a sheet with such a low additives content are therefore minimized through the successive steps of the process, embodiments of which are discussed hereinbelow.
(a) Dispersion of the reinforcing fibers in water can be performed without high dilution, that is at the normal concentrations conventionally used for cellulosic pulps in papermaking, i.e. 3% by weight and more of solid materials. The synthetic reinforcing fibers are used without the need for addition of a foaming agent or of a water viscosity modifying agent, yet in the presence of a preferably cationic dispersing agent, which preferably contains a fatty acid. The dispersing agent is preferably added to the medium in the proportion of from 5 to 15% by weight with respect to the weight of reinforcing fibers. In the case of post-lubricating, Si or Ti derivatives are preferably also added at this stage. According to one embodiment of the invention, the reinforcing fibers are continuously dispersed in step (a) and then mixed (as step (c)) in the required weight ratio with the dispersion of polymer powder from step (b).
(b) Dispersion, in parallel, of the powder elements. These are principally the thermoplastics components, i.e. the resin particles; however the dispersion may also contain non-fibrous reinforcing elements such as mineral fillers.
One advantage of the invention is that it is possible to confine the powders, even when these are very fine like certain additives, within the sheet in a homogeneous manner. Moreover, at least 98% weight of the final sheet can be comprised of the elements (resin powder and reinforcement) necessary for the final application. It has been found that this is possible owing to the ionic process which comprises first placing in suspension in water the resin and the other powdered materials, if necessary in the presence of an anti-foaming agent, then adding a few parts per thousand by weight, with respect to the powders, of an anionic material, e.g. a polycarboxylic polymer, so as to give a negative charge to the particles. After being mixed with the cationic fibrous dispersion from step (a), the charged particles consequently interact with the surface of the reinforcing fibers. In step (b) it is preferred to use anionic polycarboxylic polymers which are sufficiently hydrophobic to have an affinity towards the resin particles and sufficiently anionic to allow anionic interaction. Examples of such polymers are those containing acrylic units, such as copolymers of an acrylic acid with ethylene; or partly hydrolized polyacrylamides. With such polymers, the degree of ionicity, i.e. the carboxylic groups content, is such that the properties of said polymers in water are largely dependent on the pH. Preferably, between 0.1 and 0.5% of such an anionic polycarboxylic polymer is thus introduced, with respect to the weight of powdered material. The person of ordinary skill in the art will readily be able to determine the suitability of any particular ionic polymer additive for use in step (b), by simple tests of ionic potential or by trial and error. Various additives of this nature are commercially available, and some of those which may be employed are mentioned hereinafter in the specific Examples. Such additives typically have molecular weights in the range 1-6 million.
(c) The mixing of the two dispersions of respectively fibers and powders of opposite polarity may be carried out by any of the known mixing techniques. At this stage it is also preferred to introduce the optional polar polymers which serve to improve certain properties of the final product, dependent on the adherence of the resin to the reinforcement.
(d) In step (d), the mixed dispersions are subjected to partial flocculation. This is achieved by addition of a flocculating agent of charge opposite to that of the mixture. Such agent may be for example a cationic organic polyelectrolyte of high molecular weight (for example a cationic polyacrylamide such as a copolymer of acrylamide and a quaternary cationic monomer). The proportion of flocculating agent is preferably from 0.5 to 1.5% by dry weight of aqueous suspension. The molecular weight of the agent is preferably from 500,000 to 2 or 3 million, preferably about 1,000,000.
(e) Step (e) comprises draining the suspension and drying the semi-finished product. Draining may be carried out in typical paper making fashion by carrying the flocculated suspension on a metallic mesh or wire. Other draining techniques may equally be used. The resulting wet sheet substantially only contains (in addition to water) those elements (polymer powder and reinforcement) required for the final plastic product application, which elements are uniformly distributed throughout the thickness of the sheet and in the initially introduced proportions. After draining, the sheet is in one embodiment transferred to a supporting machine wire belt where it is dried. Preferably it is then brought, at least on its surface, to a temperature at least equal to the resin melting temperature (or in the case of a mixture, to the melting temperature of the lowest melting plastics component), so that at least on the surface of the sheet, the thermoplastics elements melt. Thus on cooling, they bond the reinforcing fibers together. The sheet then has sufficient cohesion not to require a support and it can withstand any manipulations necessary for its storage or transformation.
The dryness of the sheet discharged from the draining stage, e.g. cylinder mold or Fourdrinier machine, depends, among other things, on the granulometry of the powders used. In order to obtain a rapid and efficient drying, depending on the drying method used (infrared-radiation, high frequency radiations, hot air, belt presses or flat presses or any combination of different means), it may be advisable to conduct a wet pressing of the sheet so that densities and dryness reach a maximum. However in the case of drying by penetrating hot air, wet pressing is not generally compatible with this kind of drying as it would reduce the porosity of the sheet.
Optionally, additives can be introduced after forming the sheet, by impregnation, spraying, powdering or any equivalent means.